This invention relates to corpuscular radiation equipment in general and more particularly to an improved magnetic lens arrangement for corpuscular radiation equipment operating under a vacuum.
Magnetic lens arrangements are known for use in corpuscular radiation equipment, such as electron microscopes, which work under a vacuum. A typical prior art device includes a vacuum chamber which receives an object to be examined and at least one lens coil winding which is enclosed by a superconductive shielding device. The lens coil winding, which carries current during operation, is arranged around superconductive hollow cylindrical shielding parts which are disposed coaxial to the beam guidance axis of the lens arrangement mutually spaced one behind the other as viewed in the beam guidance direction. As a result, a gap of predetermined gap width is formed between their adjacent faces.
High voltage electron microscopes operate with beam voltages which can exceed 1000 kV. As disclosed in U.S. Pat. No. 3,916,201 it is sometimes expedient to provide several superconductive lens arrangements, disposed on a central carrier tube located in a cryostat, for such electron microscopes. In this manner a relatively compact electron microscope design is thus obtained. The lens system of known electron microscopes such as that of the aforementioned patent includes a lens arrangement of the type just described. This forms the objective lens of the lens system. It contains two hollow cylindrical shielding parts arranged one behind the other in the beam guidance direction each narrowly enclosing the beam guidance chamber. The two shielding cylinders consist of superconducting material which is kept below its transition temperature when in the operating condition through the use of a cryogenic medium such as helium. A narrow gap, in which a vacuum chamber is disposed, is formed between the adjacent faces of the shielding cylinders. An object to be examined can then be introduced into this chamber radially from the side using a special insertion device. The specimen chamber is also cooled by the cryogenic medium. Thus, while it is possible, in an arrangement such as this, to keep temperature related lateral migration of the specimen, which is referred to as thermal drift, to a minimum, e.g., to less than 0.03 nm/min, this known objective lens arrangment does not permit the examination of objects having higher temperatures, in particular objects which are at room temperature.
Each of the two shielding cylinders of the known electron microscope is surrounded by a superconducting lens coil winding which is shorted in the operating state. In other words, since there are essentially no losses in a superconductor, once a current is establisbed in a coil, the coil can be shorted and current will continue to flow. The purpose of the shielding cylinders is to have the magnetic field generated by the lens coil windings act upon the corpuscular beam only in the area of the lens gap. Thus, the faces of the two shielding cylinders facing away from each other are connected to a shielding device which also consists of a superconductive material. The shielding device encloses the outside surface and the faces of the lens coil windings. With the shielding housing designed in this manner and kept in the superconductive state, it is possible, in addition, to limit the outward spread of the magnetic field generated by the lens coil windings. It is also possible to shield the gap area in which the magnetic field acts upon the corpuscular beam against extraneous magnetic interference fields, particularly electromagnetic ac fields, to a large extent.
Since, as is known, the resolution capability of corpuscular radiation equipment depends on what is known as the apertual error constant of its lens, in particular this error constant of its objective lens, the size of the lens gap between the faces of the two shielding cylinders which are disposed opposite to each other in the known electron microscope is chosen so as to obtain a very small apertual error constant. Factors which affect the apertual error constant of such a lens arrangement include, in addition to the maximum field strength H.sub.o or the maximum magnetic induction B.sub.o in the lens gap, i.e. in the area in which the magnetic field acts upon the corpuscular beam, the field gradient along the beam guidance axis in the lens gap and thus also the design of the shielding cylinders in the area where they face each other.
These factors dictate a relatively small gap and because of this only objects cooled to the low temperature of the cryogenic medium can be examined with this objective lens arrangement. Moreover, objective lens of this type are suited only for electron microscopy utilizing what is known as the fixed beam technique. In this technique, a bunched electron beam is maintained fixed by means of magnetic fields. It penetrates the object, an enlarged picture of which is then produced by means of subsequent enlarging lens. However, this known electron magnetic microscope is not suited for what is known as a penetration scanning electron microscope. In this type of microscope, a sharply focused electron beam scans the surface of the object to be examined in accordance with a predetermined scanning scheme. If stray electrons originating from the scanning are to be registered and, if secondary electrons, as well as Auger electrons and back-scatter electrons, are to be picked up, in applicable circumstances, for additional energy dispersive radiation analysis, it is necessary that corresponding detection devices be mounted in the immediate vicinity of the object. The known objective lens design does not lend itself to such because the object chamber is too small and cannot be enlarged without either reducing the maximum field strength H.sub.o in the gap and the field gradient and thus increasing the image defects of the electron microscope, particularly its spherical and chromatic aberration, due to a corresponding enlargement of the apertual error constant.